The Cyclic AMP-GEF/C/EBP Pathway; a New PKA-independent Route for the Control of Gene Expression by Cyclic AMP

循环AMP级联是第一个解释激素如何在细胞内发挥功能变化的细胞内机制，并且仍然是典型的信号系统。直到最近，人们才认为细胞内环AMP浓度的增加仅仅通过酶“PKA”事先磷酸化转录因子“CREB”来施加基因活性的变化。然而，现在发现了一种新的途径，即环状AMP可以激活Ras和Rap蛋白。它们参与控制细胞存活、细胞周期和细胞粘附。然而，为了激活Ras和Rap，环AMP必须首先与另一类称为环AMP gef的酶直接相互作用。两个这样的循环AMP gef是EPAC或CNrasGEF。该项目的目的是确定环AMP-GEFs的激活可以导致基因表达变化的机制，在环amp依赖性中，独立于PKA。为了做到这一点，我们将使用SOCS-3基因作为范例。SOCS-3是一个诱导抑制细胞因子和生长因子受体信号传导的基因。我们最近发现SOCS-3基因也受到环AMP的正调控，通过EPAC和Rap1。我们的工作表明，这可能是通过称为C/EBP的转录因子。此外，我们还发现了一个不依赖于pka的途径，导致环AMP激活生长调节酶ERK。ERK是环AMP诱导SOCS-3所必需的，并磷酸化一类C/ ebp，同样以环AMP依赖，不依赖于pka的方式。因此，我们将描述从EPAC或CNrasGEF等环状amp - gef激活到ERK到C/ ebp的新途径，并确定其在人脐带内皮细胞（HUVECs）中激活的生物学后果。该细胞系统已被证明表达环状amp诱导的SOCS-3，并具有易于测量的优势，即通过SOCS-3介导的细胞因子信号传导抑制来确定基因诱导的生物学意义。由于环AMP-GEF-C/EBP通路是第一个环amp激活的信号通路，可以独立于经典的PKA/CREB通路控制基因表达，因此这些研究是一个迫切的科学重点。因此，我们的研究将为基因表达如何受到典型循环AMP信号系统的调节提供关键的新理解。我们在研究循环AMP信号传导的分子和细胞基础方面的共同专业知识意味着我们有能力开展这些研究。

The cyclic AMP cascade was the first intracellular mechanism described to explain how hormones exert functional changes inside cells and remains the archetypical signalling system. Until very recently in was thought that increases in the intracellular concentration of cyclic AMP exerted changes in the activity of genes solely through prior phosphorylation of the transription factor 'CREB' by the enzyme 'PKA'. However, a new route has now been discovered whereby cyclic AMP can activate proteins called Ras and Rap. These are involved in the control of cell survival, the cell cycle control and cell adhesion. However, in order to activate Ras and Rap, cyclic AMP must first directly interact with another class of enzymes called cyclic AMP GEFs. Two such cyclic AMP GEFs are EPAC or CNrasGEF. The aims of this project are to determine the mechanisms by which activation of cyclic AMP-GEFs can lead to changes in gene expression, in a cyclic AMP-dependent, independently of PKA. To do this we will use the SOCS-3 gene as a paradigm. SOCS-3 is a gene whose induction leads to the suppression of signaling from cytokine and growth factor receptors. We have recently found that the SOCS-3 gene is also positively regulated by cyclic AMP, through EPAC and Rap1. Our work shows that this is probably through transcription factors called C/EBP. Moreover, we have found that a PKA-independent pathway leads to the activation of the growth regulatory enzyme ERK from cyclic AMP. ERK is required for SOCS-3 induction by cyclic AMP and phosphorylates one class of C/EBPs, also in a cyclic AMP-dependent, PKA-independent fashion. We will therefore delineate this new pathway leading from activation of cyclic AMP-GEFs, like EPAC or CNrasGEF, through ERK to C/EBPs and determine the biological consequences of its activation in human umbilical cord endothelial cells (HUVECs). This cell system has been shown to express cyclic AMP-inducible SOCS-3 and presents the advantage that the biological significance of gene induction, as determined by SOCS-3 mediated inhibition of cytokine signalling, can be easily measured. Because the cyclic AMP-GEF-C/EBP pathway is the first example of a cyclic AMP-activated signalling pathway that can control gene expression independently of the classical PKA/CREB route, these studies are an urgent scientific priority. Our investigations will therefore provide a critical new understanding of how gene expression is regulated by the prototypical cyclic AMP signalling system. Our joint expertise in studying the molecular and cellular basis of cyclic AMP signalling means that we are well equipped to carry out these investigations.